For many years it has been known to biaxially stretch or molecularly orient webs of thermoplastic materials both to produce films of desired thickness and also to improve the mechanical and dimensional properties of the film. Examples of equipment used for such stretching or orienting are found in U.S. Pat. Nos. 3,375,313 and 3,676,539. Certain usually undesirable characteristics of such stretched films have been observed over the years, such as the tendency of some films to acquire a set curvature in the longitudinal direction following winding onto a core or spool, a phenomenon known as core-set as discussed at length in U.S. Pat. No. 4,141,735. Such films also frequently exhibit a tendency to curl in their longitudinal direction or to camber transversely, depending on the particular thermoplastic material used and the process used for biaxial stretching.
A variety of techniques have been developed in efforts to minimize or at least control core set, longitudinal curl and transverse camber. British Patent 1,030,288 discloses a method for making polymeric strap in which it is recognized that differential heating of a film during stretching and differential cooling of the film following stretching can contribute to camber and core set. U.S. Pat. No. 3,429,961 discusses a process for improving the uniformity of several film properties in which a temperature profile is established across the transverse width of the film just prior to initial stretching; so that, following initial stretching, the film has convex upper and lower surfaces. Stretching in the perpendicular direction to the initial stretch then produces a flat film having more uniform properties. U.S. Pat. No. 3,632,726 discloses an apparatus and method for making dimensionally stable plastic film, in which a previously oriented film is moved on cushions of air between a heat source and a heat sink, thereby producing a film of improved flatness.
In U.S. Pat. No. 3,796,785, heated feed and delivery rollers are described which rotate about parallel axes, with the temperature of the feed roller being below the softening temperature of the unstretched material and the temperature of the faster rotating delivery roller being below the softening temperature of the stretched material but well above the softening temperature of the unstretched material. The arrangement causes stretching of the material in a narrow gap between the rolls and is said to ensure films of more uniform thickness. U.S. Pat. No. 3,816,886 discloses an apparatus for longitudinally stretching thermoplastic film in which the film is drawn under stretching tension over a first, heated roller and then over a second, cooled roller very closely adjacent the first roller, while infrared heat is applied to the film between the rollers to facilitate stretching. Canadian Patent 954,680 discloses an apparatus for production of crimped filaments of plastic film in which a film under stretching tension is passed between rollers having different temperatures; so that, there is a temperature gradient across the thickness of the film while it is passing through the nip of the rollers. The patentee claims that the molecular orientation of one surface of the web thus is modified by the rollers to a different extent than the orientation of the other surface, as the web is stretched at or near the nip of the rollers.
U.S. Pat. No. 4,165,356 discloses a method and apparatus for forming tubular plastic material in which an extruded tube is subjected to cool air flow through its interior and warmer air flow over its exterior during longitudinal and circumferential stretching. The temperature differential is said to cause the edges of the tube to flare outwardly upon severance. U.S. Pat. No. 4,293,508 describes an apparatus for longitudinally stretching polyethylene terephthalate film in which heating of the film during stretching is carried out in two stages, an initial infrared preheating and a final, concentrated infrared heating to a temperature suitable for elongation, leading to a film said to have improved flatness and surface quality.
While processes and apparatus of the types disclosed in such patents have contributed to the ability of those skilled in the art to produce biaxially oriented films with significantly improved characteristics, a need has continued to exist for a reliable process for controlling curl in films made from thermoplastic material. Particularly in processes in which the film is produced by stretching a substantially flat sheet having beaded edges, the ability to produce films having a desired degree of curl has been difficult to achieve.